Joshua 19:9

‘ Out of the part of the children of Judah was the inheritance of the children of Simeon, for the portion of the children of Judah was too much for them. Therefore the children of Simeon had inheritance in the midst of their inheritance.'

Coming after verse 8 this is probably the writer's explanation added to the official record. It confirms that because too much had been allocated to Judah, Simeon were allotted part of their portion. In view of the fact that all was given by lot under YHWH's direction no one would later have dared suggest such an idea unless it had been so.